PROJECT SUMMARY Tregs and MDSCs have emerged as key suppressor cells that inhibit effector immunity in the TB granuloma. In addition, Tregs have been implicated in the maintenance of HIV latency. We have generated two selective biologic agents for potent depletion of Tregs and and MDSCs using diphtheria toxin (DT) fusion protein technology: DT-IL2 targets IL2-R-positive (CD25+) cells (Tregs), and DT-IL4 targets IL4-R positive (CD124+) cells (MDSCs). Our preliminary data demonstrate that: (i) DT-IL2 and DT-IL4 reduce the abundance of Treg and MDSC cells, respectively, in mouse lungs and spleens, (ii) DT-IL2 and DT-IL4 reduces the ability of M. tb to proliferate in an acute murine TB infection model, (iii) the drug, tasquinimod which has potent anti-MDSC activity, reduced the ability of M. tb to proliferate in an acute TB infection model with a concomitant reduction in MDSCs, and (iv) administration of DT-IL2 to CD4+ T cells from patients with controlled HIV reactivates viral replication. These data support the hypotheses that (1) Tregs and MDSCs play important inhibitory roles in M. tb containment, and (2) that Treg cells play a role in HIV latency maintenance. A central scientific premise of this application is that further definition of the immunosuppressive roles of Tregs and MDSCs in TB and HIV infection will reveal important immunopathogenesis mechanisms that may be exploited towards developing improved host-directed therapies (HDTs) for both diseases. In this proposal we seek to address this premise by pursuing the following aims. Aim 1: define the role of Treg cells in the TB granuloma using targeted CD25+ cell depletion with DT-IL2. We will (a) evaluate the cellular composition of both cellular (C57BL/6, WT which are sst1R) and necrotic (C57BL/6 sst1S) murine granulomas with and without CD25+ cell depletion using single-cell RNA-Seq (SSRS) as well as traditional methods, (b) evaluate the efficacy of targeted CD25+ cell depletion in chronic murine TB with and without anti-TB drug therapy, and (c) evaluate the contribution of Treg cells to the process of cavitation (granuloma breakdown) using a novel model of rabbit cavitary TB. Aim 2: define the role of MDSCs in the TB granuloma using targeted CD124+ cell depletion with DT-IL4. We will (a) evaluate the cellular composition of both cellular and necrotic murine granulomas with and without CD124+ cell depletion using SSRS as well as traditional methods, (b) evaluate the efficacy of targeted CD124+ cell depletion in chronic murine TB with and without anti-TB drug therapy, (c) assess the impact of dual CD25+ and CD124+ cell depletion in the TB granuloma with B6 sst1R and sst1S mice using SSRS as well as traditional methods, and (d) evaluate the efficacy of dual CD25+ and CD124+ cell depletion in chronic murine TB with and without anti-TB drug therapy. Aim 3: conduct translational applications of CD25+ cell depletion with (i) BCG vaccination and (ii) HIV reactivation. We will (a) evaluate the efficacy of targeted CD25+ cell depletion as a vaccine adjuvant prior to BCG vaccination and (b) evaluate the impact of targeted CD25+ cell depletion on HIV reactivation in PBMCs from patients with controlled HIV infection.